High in specific strength and specific modulus, carbon fiber reinforced composite materials (hereinafter occasionally referred to simply as composite materials) in recent years have come into wider use in various fields including aircraft, automobiles, sports tools, fishing tools, blades for wind power generation, and personal computer housing. These structures often have complicated shapes and many of them are produced by laminating layers of carbon fiber reinforced fiber woven fabric prepreg that is in the form of thin, flexible sheets with isotropic material properties.
These prepreg sheets are generally produced from thermosetting resin compositions, epoxy resin compositions among others, that have good mechanical properties, high heat resistance, and high process-related handleability. In the field of manufacturing of structural members of aircraft and automobiles, there are increased needs in recent years for materials with improved mechanical properties that enable weight reduction and enhancement in major characteristics including tensile strength and compression strength has been strongly called for. In increasing the tensile strength of composite materials produced from epoxy resin compositions, not only an improvement in the tensile strength of the carbon fibers used as the base material, but also a reduction in the crosslink density in the cured products of the epoxy resin compositions (hereinafter referred to simply as cured products) is effective. However, a reduction in crosslink density leads to the problem of poor heat resistance of the cured product. On the other hand, a cured product with a high elastic modulus can work effectively in producing a composite material with an improved compression strength. However, a high crosslink density is an effective factor in producing a cured product with an improved elastic modulus. Thus, it has been a very difficult issue to produce a composite material that is high in both tensile strength and compression strength.
As a prior art technique, Patent document 1, for example, proposes the use of diaminodiphenyl sulfone, 3,3′-diaminodiphenyl sulfone in particular, as a curing agent for epoxy resins, is effective for producing a cured product with an improved elastic modulus, but there are no descriptions about providing a composite material with an improve strength. Patent document 2 describes that the use of 3,3′-diaminodiphenyl sulfone and 3,4′-diaminodiphenyl sulfone serves to producing a cured product with an improved elastic modulus, which enables the production of a composite material with an improved compression strength, but there are no descriptions about improving the tensile strength of composite materials. To produce a composite material with an improved tensile strength, decreasing the number of functional epoxy groups is effective because it works to reduce the crosslink density. To provide a composite material with an improved compression strength, on the other hand, increasing the number of functional epoxy groups is effective because it works to raise the crosslink density. Furthermore, the introduction of molecules having a structure that is bent or a structure that enables easy interaction of molecular chains is considered to be effective for producing a cured product with an improved elastic modulus. For example, disclosed examples include a resin composition containing a t-butyl catechol type epoxy resin (Patent document 3). Its molecule has a structure in which the number of epoxy groups is smaller to enable easy interaction of molecular chains and contains hydroxyl groups that are expected to help form hydrogen bonds. Patent document 4 discloses an epoxy resin having a t-butyl hydroquinone structure and Patent document 5 discloses an epoxy resin composition having a resorcinol structure. Patent documents 3 to 5, however, fail to provide a composite material having a required strength and heat resistance. Patent document 6 proposes a composition containing a t-butyl catechol type epoxy resin to be used as a component for painting material. However, required mechanical properties were not achieved when simply using it as a resin composition to produce a composite material.